1. Field of the Invention
The present invention relates to an improved ion measuring composite electrode, a measuring instrument incorporating such a composite electrode, and manufacturing method for a double glass pipe.
2. Description of Related Art
When measurement is performed for ion concentration of a sample, such as a pH value in a liquid, there has been used an ion measuring composite electrode constructed, as an integral one piece of a glass pipe serving as a measuring electrode and a glass pipe installed for a reference electrode. Generally, since an ion measuring composite electrode is relatively complicated in structure the resulting electrode can be relatively large in size, and it is difficult to manufacture an equivalent electrode that is small in size.
FIG. 4 is a schematic view showing a construction of a pH composite electrode 20 employing a double pipe structure, in which a responsive section for an ion concentration, such as a pH value, is formed as a long and narrow channel, and which has been developed to enable an ion concentration of a very small amount of a solution, such as a measuring specimen, to be measured. Furthermore, FIGS. 5A and 5B are views describing a manufacturing method for this type of double pipe structure.
In FIGS. 4, and 5A and 5B, a numerical symbol 2 denotes an outer pipe and 3 denotes an inner pipe of the double pipe structure. The outer pipe 2 is formed so that a distal end 2a thereof is sealed and is of an extremely narrow diameter, in the order of several mm, wherein a diameter of the proximal end 2b assumes a large value so as to be connectable to a body 20a side of the pH composite electrode 20. Furthermore, a pH responsive section 4 made from a glass that is responsive to an ion, such as a pH value, is provided at the distal end 2a of the outer pipe 2.
The distal end 3a of the inner pipe 3 is expanded at an outer diameter thereof and the expanded outer periphery is glass-welded to the inner peripheral surface of the outer pipe 2 to thereby create a separate space 5 in the interior of the inner pipe 3 for communicating with a pH responsive section 4. The space 5 extends through the inner pipe 3 from an annular space 6 formed between the outer pipe 2 and the inner pipe 3 with the result that a construction is formed so that the space 5 can be filled with a measuring electrode internal liquid 7 and the space 6 can be filled with a reference electrode internal liquid 8.
A liquid connecting section 9 made of a ceramic filling a small hole is formed in the outer pipe 2, and is located near the lower end of the annular space 6 to communicate with the outside of the outer pipe 2. An internal electrode 10 is immersed in a measuring electrode internal liquid 7 in the space 5, and a reference electrode 11 is immersed in the reference electrode internal liquid 8 in the space 6. Therefore, when a pH composite electrode 20 is constructed, a measurement of a pH value is enabled for even a very small amount of a measuring specimen when the liquid connecting section 9 and the pH responsive section 4 is immersed in the solution, as a measuring specimen.
In the fabrication of a double pipe structure with the outer pipe 2 and the inner pipe 3, as shown in FIG. 5A, a technique has been adopted in a prior art practice where the opening at the distal end 3a of the inner pipe 3 is expanded so that the outer periphery of the opening becomes close to the inner peripheral surface of the outer pipe 2 to form a flange section 12. The inner pipe 3 thus worked is inserted into the outer pipe 2 with the distal end 3a as a leading head. At this time, the proximal end 3b of the inner pipe 3 is carefully fixed with a bushing or the like (not shown) so that the distal end 3a is aligned with the outer pipe 2a with respect to the center axes. Then, heat is applied on the outer peripheral surface of the outer pipe 2 using a gas burner 17 while rotating the outer and inner pipes 2 and 3 using a lathe or the like to thereby weld the flange section 12 to the inner peripheral surface of the outer pipe 2.
In such a welding, however, a problem of a defective welding has arisen as shown in FIG. 5B since the inner pipe 3 can move to create a positional instability in the outer pipe 2 so that the outer pipe 2 and the inner pipe 3 are insufficiently aligned with each other with respect to the center axes at the distal ends 2a and 3a thereof.
Also a problem of transferring more heat than necessary from the gas burner 17 to the flange section 12 in those portions where the inner pipe 3 is closer to the outer pipe 2 can occur, which can cause fusion of the glass over a wide range to cause the outer pipe 2 to be placed in wide contact with the inner pipe 3 in a portion 12a apart from a target portion. Additionally, in those portions where the inner pipe 3 is positioned farther away from the outer pipe 2, welding can produce a clearance 12b, which can permit the spaces 5 and 6 to communicate with each other. This problem can occur since heat from the gas burner 17 is hard to be transferred to the flange section 12.
In addition, since the outer and inner pipes 2 and 3 are formed in an extremely narrow offset, difficulty has accompanied the aligning of both the center axes. That is, not only is there a shift in a center axis between the distal ends 2a and 3a which is hard to visually recognize from outside the outer pipe 2, but difficulty also arises in visual judgment on a value of a force imposed on a site where the flange section 12 is in contact with the inner peripheral surface of the outer pipe 2, if any. Hence, even though the welding work is done with deliberate attention thereto and with extra time, a case can arise where a shift between the center axes is occurs, and defective weld happens, resulting in a poor production rate.
Moreover, a necessity exists for the interiors of the inner and outer pipes of the formed double pipe structure to be completely filled with an internal liquid, and with a decrease in diameters of the outer and inner pipes 2 and 3, the space 6, which is the gap between the outer and inner pipes 2 and 3, can become too narrow; therefore, the reference electrode internal liquid 8 can be difficult to evenly spread within the space 6 to every part thereof and a bubble can be produced between the reference electrode 11 and the liquid connecting section 9 to cause an electrically open state, leading to a problem of disabling a measurement. In a case where the reference electrode internal liquid 8 decreases, a bubble can also be incorporated into the space 6 when the reference electrode internal liquid is supplemented.
Thus the prior art is still seeking both an improved method of manufacturing concentric small diameter glass tubes and a resultant improved ion measuring composite electrode with improved production rates.